Psychologists have opined that diet drugs work since they make people 'eat healthy'.
In the study, presented at the British Psychological Society's Division of Health Psychology conference in Birmingham, researchers found that dieters who lost the most weight on the drugs had also reduced the amount of fatty junk food they ate.
However, some people reacted differently to starting the drugs, taking them as a license to eat more unhealthy food such as crisps, reports The Telegraph.
To reach the conclusion, researchers analysed data of 572 people who had been prescribed the diet drug orlistat by their doctor.
The drug works by reducing the amount of fat absorbed by the body.However, this fat is them eliminated in bowel movements, which can cause disagreeable side effects.
Amelia Hollywood, a PHD student at the University of Surrey and one of the researchers who carried out the study, said: "Our findings support the idea that orlistat works not only on a physical level, but also psychologically - as it encourages people to see their diet as a cause of their weight problem.
"In addition, the side effects are so unpleasant that people avoid bad eating fatty foods and therefore lose weight.
"However, the way in which some people responded to orlistat was surprising.
"Some participants in this study reported that their eating behaviour became significantly unhealthier over the six month period."
She added: "People also told us that they were not adhering to the medication as they should. It seemed that these people were taking orlistat as a lifestyle drug - choosing to take it when they were eating foods higher in fat to reduce any weight gain or not taking it when going on holiday or out for a meal as they didn't want to experience the consequences of eating fatty foods."
The preliminary findings found that on average those taking the diet pills lost almost 10lb over six months.